From eed530b6c67624db3f2cf477bac7c4d005d8f7ba Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Wed, 2 May 2012 13:30:03 +0000 Subject: tcp: early retransmit MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This patch implements RFC 5827 early retransmit (ER) for TCP. It reduces DUPACK threshold (dupthresh) if outstanding packets are less than 4 to recover losses by fast recovery instead of timeout. While the algorithm is simple, small but frequent network reordering makes this feature dangerous: the connection repeatedly enter false recovery and degrade performance. Therefore we implement a mitigation suggested in the appendix of the RFC that delays entering fast recovery by a small interval, i.e., RTT/4. Currently ER is conservative and is disabled for the rest of the connection after the first reordering event. A large scale web server experiment on the performance impact of ER is summarized in section 6 of the paper "Proportional Rate Reduction for TCP”, IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf Note that Linux has a similar feature called THIN_DUPACK. The differences are THIN_DUPACK do not mitigate reorderings and is only used after slow start. Currently ER is disabled if THIN_DUPACK is enabled. I would be happy to merge THIN_DUPACK feature with ER if people think it's a good idea. ER is enabled by sysctl_tcp_early_retrans: 0: Disables ER 1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4. 2: (Default) reduce dupthresh like mode 1. In addition, delay entering fast recovery by RTT/4. Note: mode 2 is implemented in the third part of this patch series. Signed-off-by: Yuchung Cheng Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'net/ipv4/tcp.c') diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 9670af341931..6802c89bc44d 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -395,6 +395,7 @@ void tcp_init_sock(struct sock *sk) tp->mss_cache = TCP_MSS_DEFAULT; tp->reordering = sysctl_tcp_reordering; + tcp_enable_early_retrans(tp); icsk->icsk_ca_ops = &tcp_init_congestion_ops; sk->sk_state = TCP_CLOSE; @@ -2495,6 +2496,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level, err = -EINVAL; else tp->thin_dupack = val; + if (tp->thin_dupack) + tcp_disable_early_retrans(tp); break; case TCP_REPAIR: -- cgit v1.2.3